1. Field of the Invention
This invention relates to transceivers for reflector-type antennas, and more particularly to hand and polarization-selectable transceivers for a reflector-type antenna.
2. Description of the Related Art
Reflector-type antennas use a main reflector, a curved surface typically with the cross-sectional shape of a parabola, to direct RF energy. Reflector antennas have some of the highest gains and produce the narrowest beamwidths of any antenna type. To achieve narrow beamwidths, the main reflector must be much larger than the RF wavelength so reflective antennas are typically used in the highest frequency portion of the RF spectrum.
The operating principle of a reflective antenna is that a point source or “feed” of RF energy at the focal point of the main reflector will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the local point. The feed antenna is typically a low-gain type such as a half-wave dipole or a feed horn. The feed antenna is connected to the RF transceiver including a transmitter and a receiver by means of a waveguide or coaxial cable. Some configurations include a sub-reflector between the feed and the main reflector to allow the feed to be located at the vertex of the main reflector or to shape the feed-horn pattern.
Reflector antenna feeds are typically designed for single frequency bands of operation. Transmission and reception via a single antenna may be accomplished using time division multiplexing implemented with a waveguide switch. Another approach is to use a polarization duplexer, also known as an orthamode transducer (OMT), which allows for simultaneous transmission and reception within the same band at orthogonal polarizations. An OMT serves either to combine or to separate two orthogonal polarized RF signal paths. OMTs may be used with a feed horn to isolate orthogonal polarizations of a signal and to transfer transmit and receive signals to different ports.
Reflector antenna feeds may be designed for dual frequency bands of operation in which the transmit band and receive band are fixed. A frequency duplexer allows for simultaneous transmission and reception in the two separate bands. A frequency duplexer serves either to combine or to separate two RF signal paths in the different bands. A limitation of this approach is that the paired antenna must be hardwired or “gendered” to transmit and receive in the opposite bands.
The Defense Advanced Research Projects Agency (DARPA) issued a broad agency announcement (DARPA BAA) 12-23 on Feb. 10, 2012 directed to “Mobile Hotspots”, in which it announced a need for technology to support an ad hoc communications network that can be put over a battlespace at a moments notice with the capacity and range needed to connect remote and mobile warfighters with forward operation bases, higher-echelon tactical operations centers, and remote intelligence, surveillance, and reconnaissance sources, and fixed communications infrastructure A steerable bi-directional E-band Transceiver with Pointing. Acquisition and Tracking (PAT) is part of Phase 1 Technology Development. The E-band communication band is split into two “low and high” sub-bands of approximately 71-76 GHz and 81-86 GHz. Either time-domain diversity (TCC) or frequency-domain diversity (FDD) for separating transmit and receive functions may be considered. DARPA considers this to be “technology hard”.
Early systems using the new E-band frequency allocation have used a “hardwired.” or “gendered” topology to transmit and receive from a specific low and high band frequency combination, requiring transceivers an opposite sides of a communication link to be matched. In a mobile ad hoc communication network such as contemplated by DARPA, a gendered solution would require two different types of transceivers at each platform.
In order to pair any transceiver with any other transceiver in the network, the transceiver must be “genderless” or “gender-selectable”. Any gender-selectable transceiver, by definition, must have some form of front-end RF switching device. For example, a TDD (Time-Domain Duplexed) system must be capable of switching the antenna between the transmitter and receiver at extremely fast rates (requiring an electronic switch). A FDD (Frequency-Domain Duplexed) system must be capable of switching the transmitter and receiver between two different transmit and receive bands. Here the switching is done before the connection is made and can be done with a slower electro-mechanical switch. A drawback of duplexed systems is the added front-end RF loss due to the switch. For instance, the best high power electronic E-band waveguide switch for TDD has about 3 dB of loss. The best high power electro-mechanical E-band waveguide switch suitable for FDD has about 0.8 dB of loss.